Various hybrid powertrain architectures are known for managing the input and output torques of various prime-movers in hybrid vehicles, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to a battery pack. The internal combustion engine in a series hybrid is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack. Parallel hybrid architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and multi-mode operation.
It is known to select engine power from the road-load power required plus an additional quantity of engine power based on the energy storage system's (e.g. battery's) state-of-charge. Following selection of engine power, the engine's optimal fuel economy or optimal emissions map or a combination thereof may be used to select the engine's torque/speed operating point. The battery power effected is that which is required, in combination with the engine power, to meet the road-load power requirements and to compensate for power losses within the system.
Known systems do not optimize the power flow of all the propulsion system components. Typically, only the engine operation is optimized. The prior art does not weigh additional factors such as other system mechanical and electrical losses and battery usage factors in selecting the overall system's preferred operating point.
A preferred method for determining input speed is disclosed in commonly assigned and co-pending U.S. patent application Ser. No. 10/686,508 and Ser. No. 10/686,034. Therein, preferred operating points for a vehicle powertrain including an engine and a transmission are determined in accordance with a comprehensive operational mapping of input and output conditions and corresponding aggregate system losses corresponding to engine and transmission losses. In a hybrid transmission application, additional losses from motors and batteries are aggregated into the system losses and battery constraints are considered in determining preferred operating points. Preferred operating points are provided in one or more sets of minimized data for on-vehicle implementation. Desired input speed is provided by the system controller, for example in accordance with a desired operating point of the engine to meet various efficiency and emission objectives.
A preferred speed control for a hybrid transmission is described in detail in commonly assigned and co-pending U.S. patent application Ser. No. 10/686,511. Therein, a multi-mode hybrid transmission is described having speed control provided via an open loop model derived as a function of preselected transmission accelerations and controlled and uncontrolled transmission torques. Motor torques are selected as the controlled torques and other preselected transmission torques are selected as the uncontrolled torques. The control also employs a closed loop control effort responsive to at least one preselected transmission speed error.
A preferred hybrid EVT powertrain system and method of its operation which considers the system as a whole in determining operating conditions is described in detail in related, commonly assigned and co-pending U.S. patent application Ser. No. 10/779,531. Optimum or preferred system operating points for a preselected powertrain operating parameter are determined through comprehensive consideration of engine, mechanical and electrical based contributory system losses. These operating points are identified by determining a feasible operating space for the preselected powertrain operating parameter, searching the feasible operating space for a value corresponding to a minimum system power loss, and establishing the preferred operating point as the value corresponding to the minimum system power loss. Preferably, the preselected powertrain operating parameter is input torque. Because of the central role of the energy storage system in a hybrid electric vehicle powertrain and the known constraints on operational life due to battery utilization and current throughput associated with repetitive charge/discharge cycles, it is desirable that the method of selecting the operating points of the powertrain corresponding to a minimum system power loss also include other empirical factors not related to actual power loss but effective to bias the minimum power loss away from input torques that are less desirable because of other considerations, such as battery use.
Therefore, it is desirable to develop a hybrid vehicle powertrain system and method of operation which incorporates selecting the operating points of the powertrain corresponding to a minimum system power loss and which also is adapted to bias the selection as a function of parameters associated with the use of the energy storage system.